In many industrial manufacturing processes it is particularly important to know the net weight of packed goods (e.g. bulk goods, foodstuffs, etc.). When the tare weight is negligibly small compared with the net weight, gross weighing of the packed goods is generally sufficient. However, if the wrappings or packaging are very small, as in the case of pharmaceutical formulations containing active substances, such as capsules, for example, the ratio between the surface area and the volume is shifted considerably towards the surface area, and the tare weight is easily a multiple of the net weight. In such cases, fluctuations in the tare weight cannot be disregarded. The net weight must therefore be determined either by directly weighing the net contents or by weighing the object to be measured before and after packaging (determining the tare and gross weights).
Naturally, net weighing will always be carried out on the filling or packing machine where the actual metering process takes place. The same is also true, however, of gross/tare weighing when operating at high production rates, as otherwise the proper association between the tare and gross measurements of the individual items could only be achieved with difficulty or at considerable expense.
When high production rates (e.g. 100,000 items/hour or more) are required while at the same time small quantities in the mg range are being weighed out, the measuring apparatus is subject to extreme demands in terms of precision and speed. For example, online gross/tare weighing systems on capsule filling machines in which a capacitive measuring system is used are known in the prior art. The substance to be measured is passed through a capacitor before and after filling and the mass introduced is calculated from its change in capacity. However, it has been found that processes of this kind are only suitable for use with amounts of more than 50 mg. Modern gravimetric weighing cells meet significantly higher accuracy requirements. However, they operate at a low speed of only a few capsules per second. Significantly higher throughput speeds could be achieved by providing weighing cells of this kind in parallel, but gravimetric weighing cells have the limitation of being highly sensitive to vibrations and having to be operated in cycles rather than continuously. Consequently, gravimetric weighing cells can only be used to a very limited extent on continuously operating capsule filling machines.
There is thus a need for a weighing method which—when detached from the filling machine and operating as a stand-alone solution—is capable of determining the contents of filled and sealed capsules.